Research provides new insights into actinide electronic structure

A team of U.S. Department of Energy (DOE)
researchers from the Lawrence Livermore, Lawrence Berkeley, and Los Alamos
national laboratories and SLAC National Accelerator Laboratory, studying the
fundamental properties of the actinide elements, has significantly advanced the
understanding of the electronic structure of elements that have electrons
occupying f-orbitals.

Using a synchrotron-based X-ray spectroscopic
tool known as resonant X-ray emission spectroscopy (RXES), the team measured
X-ray spectra for a large number of uranium (U) and plutonium (Pu)
intermetallic compounds. (The actinides include the 15 metallic chemical
elements with atomic numbers from 89 to 103, actinium through Lawrencium.)

The results show that the actinide atoms in
many of the intermetallic compounds and pure a-phase uranium and plutonium
metals exhibit multiconfigurational electronic structures. Such structures
cannot be described as a single state with a fixed number of f-orbital
electrons; rather, they must be described using a mixture of states with
different numbers of f-electrons (for Pu, f4, f5, f6; for U, f1, f2, f3).

The 5f electrons in U and Pu sit on the edge
between being strongly bonding with ligand spd-states (itinerant electrons) and
residing close to the nucleus (localized electrons). The unusual properties of
these elements and their compounds (the six different allotropes of elemental
plutonium) are widely believed to depend on f-orbital occupancy and the degree
of electron delocalization; however, before now, there has been no way to
quantitatively determine these parameters.

The new results provide that means, and
should provide a strong experimental basis for building a new framework for
understanding the behavior of strongly correlated electrons in actinide
materials.

The research appears in the Proceedings of
the National Academy of Sciences.